Certain optical systems operable to change the intensity of light applied to a unit surface area are known. A representative system is depicted schematically in FIG. 4, comprising (on an optical axis A) a light source 111, a convex lens 112, and a stop 113 defining an aperture 113a. Light flux is directed onto the surface of a specimen 118. The light flux emitted by the light source 111 is focused by the lens 112 at a focal point FP located between the stop 113 and the specimen 118; downstream of the focal point FP, the light flux diverges. With the FIG. 4 apparatus, the intensity of light per unit surface area of the specimen can be changed by moving the specimen 118 along the optical axis A (i.e., moving the specimen left or right as indicated by the arrows beneath the specimen).
Optical systems such as that in FIG. 4 are conventionally used, e.g., in testing schemes for ascertaining the durability of various materials when exposed to laser light. Such tests are increasingly indicated as lasers become increasingly more powerful. The tests are typically performed by irradiating the specimen with a laser while critically observing and evaluating any damage caused by the exposure. Usual tested specimens include optical components such as lenses and mirrors, as well as coatings applied to such optical components for, e.g., antireflective purposes.
In an apparatus as shown in FIG. 4, as light intensity per unit surface area is changed, the irradiation surface area of the laser light also changes. In other words, one cannot change the intensity of light per unit surface area of the specimen without also changing the surface area of the specimen that is irradiated by the laser light. Consequently, this kind of optical system cannot be used if one wishes to limit or otherwise control the size of the irradiation field on the specimen surface.
Another testing apparatus as known in the art is schematically depicted in FIG. 5. A high-power laser light source 211 produces light that passes through a condensing optical subsystem 220 onto the surface of a specimen 218. A beam splitter 215, situated between the condensing optical subsystem 220 and the specimen 218, reflects some of the laser light toward a power monitor 217. The power monitor 217 measures the intensity of laser light at the specimen surface based on the reflected light. An extinction filter 219 is situated between the laser light source 211 and the condensing optical subsystem 220. The intensity of laser light at the specimen surface can be changed by changing the extinction filter 219 as appropriate for the particular specimen.
Apparatus as shown in FIG. 5 also have substantial shortcomings. First, having to change the extinction filter 219 to change light intensity is extremely troublesome. Second, if the light source 211 is an excimer laser or other laser that emits a very high-energy laser light, extinction filters capable of withstanding such intense light are rare or do not exist. Third, changing light intensity by changing the extinction filter means that the intensity of the light per unit surface area can be changed only step-wise. This makes it impossible to randomly set the value of the light intensity, i.e., to change the light intensity continuously over a range.